Multi-channel cellular communications intercept system

ABSTRACT

A multi-channel cellular communications intercept system for monitoring and then intercepting communications between a mobile unit and a base station in one cell of a cellular telephone system. The system includes a controller and a plurality of receivers with each receiver monitoring the forward control channel from a different cell within a geographical area of interest. When a global page for a target mobile unit is received, one of the receivers in the system is retuned to the forward voice communication channel on which the target mobile unit will be broadcasting for the purpose of monitoring and/or recording the conversation. The remaining receivers are retuned to the strongest forward control channels in the area.

FIELD OF THE INVENTION

This invention relates to a cellular communications intercept systemincluding a plurality of receivers under control of a system controllerfor intercepting, monitoring and/or recording cellular telephoneconversations.

BACKGROUND OF THE INVENTION

Recent advances in communications technology have created a burgeoningtelephony market, both in terms of the availability of new services andnew forms of equipment. Cellular telephone technology is one of thosenew services now being routinely offered in many locations across thecountry. While cellular telephony is a powerful and efficient newmedium, it has also become an important tool for carrying out illegalactivities. The mobility and perceived privacy of cellular mobilecommunications holds significant appeal to those engaged in illegalactivities. In fact, in many areas drug dealers routinely use cellulartelephony to conduct their day-to-day illegal drug deals.

A prior art cellular system 10 is illustrated in FIG. 1. The cellularsystem 10 consists of a frequency-modulation (FM) radio network coveringa series of geographical areas referred to as cells and identified byreference character 12 in FIG. 1. The two-way radios in each mobileunit, commonly referred to as cellular telephones, operate within thecells 12 by communicating between the mobile unit 14 and a base station16 within each cell 12. The cellular system 10 is defined by a pluralityof base stations 16 distributed over a geographical area of systemcoverage and managed and controlled over links 17 by a centralizedswitch referred to as the mobile telephone switching office 18 (MTSO).The base station 16 within each cell 12 has the responsibility forcontrolling and communicating with all mobile units 14 within the celland for relaying voice traffic between the mobile units 14 and themobile telephone switching office 18. The MTSO 18 then relays the voicetraffic to the public service telephone network 20 (PSTN) over a link19.

There are four frequencies used between each base station 16 and anymobile unit 14. Two forward frequencies are used for communication fromthe base station 16 to the mobile unit 14 and two reverse frequenciesare for communications from the mobile unit 14 to the base station 16.One forward frequency is paired with one reverse frequency forcommunicating control and status information between any mobile unit 14and the base station 16. A second forward frequency is paired with asecond reverse frequency for communicating voice and data between thebase station 16 and the mobile unit 14. By convention, these variousfrequencies are referred to as the forward control channel, the forwardvoice channel, the reverse control channel, and the reverse voicechannel, respectively. Each cell 12 has a single forward control channeland a single reverse control channel for all control and status messagesbetween mobile units 14 within the cell and the base station 16 of thecell. Each cell 12 has a number of forward and reverse voice channelsfor assignment by the base station 16 as required to meet thecommunications traffic demands. The forward voice channel is forcommunicating voice and data from the base station 16 to the mobile unit14, while the reverse voice channel is the frequency on which voice/datais passed from the mobile unit 14 to the base station 16.

The following is an example of the technique for setting up an externalcall in the cellular system 10. When the user of the mobile unit 14first turns on his cellular telephone, the unit scans all forwardcontrol channels to determine the strongest one. Recall that each cell12 has a base station 16 and each base station has a unique forwardcontrol channel. The mobile unit 14 then locks onto the strongestreceived forward control channel, presumably the forward control channelfor the cell 12 in which the mobile unit 14 is located, and continues tomonitor it.

When a call is placed to a mobile unit 14 from the conventionaltelephone network, a global page message (or MTSO page), which includesthe telephone number or electronic serial number of the called mobileunit, is generated at the mobile telephone switching office 18. Theglobal page is then transmitted in digital format to the base station 16of every cell 12 that is within the mobile telephone switching office18. This multiple transmission occurs because the mobile telephoneswitching office 18 does not know where the target mobile unit 14 is, orif it is within the range of the mobile telephone switching office 18.Each base station 16 then transmits the global page on its uniqueforward control channel. When the activated mobile unit 14 identifiesits own telephone number within the global page, it responds to the basestation 16 on the reverse control channel, basically saying, "Here Iam". The called mobile unit 14 responds on the dedicated reverse controlchannel that is unique to the cell 12 in which it is located, andtherefore none of the other base stations 16 will see a response to theglobal page. The base station 16 that is tuned to that reverse controlchannel sees the response and selects a forward voice channel/reversevoice channel assignment for the mobile unit 14. A command (referred toas a base station page) is then transmitted to the mobile unit 14 fromthe base station 16 in the form of a voice channel assignment, "I hearyou, and please tune to channel x to execute your call". Upon receivingthis message, the mobile unit 14 tunes its receiver to the designatedforward voice channel to hear the called party and tunes to the pairedreverse voice channel to transmit.

This technique for call set up is further complicated as the mobile unit14 passes from one cell to another during the conversation. Thecontrolling base station 16 continually monitors the strength of thesignal from the mobile unit 14 and if that strength diminishessignificantly (perhaps indicating that the mobile unit 14 has entered adifferent cell) then a cell hand-off occurs. To accomplish this, a datamessage is transmitted from the base station 16 on the forward voicechannel telling the mobile unit 14 to tune to a different voicefrequency, one that is controlled by a neighboring cell. The mobile unit14 retunes and the conversation continues on the new frequency. Manycell hand-offs may occur during a single conversation as the mobile unit14 exits and enters cells.

When the call is completed, the mobile unit 14 scans and locks onto thestrongest forward control channel, which again indicates the cell 12 inwhich the mobile unit 14 resides and listens for a global page thatcontains its telephone number or electronic serial number.

As discussed above, the mobile unit 14 receives on the forward voicechannel and transmits on a different frequency, the reverse voicechannel. This would imply that one would have to monitor bothfrequencies to hear both sides of the conversation. Such is not thecase, however. Because it is unnatural for the telephone user to speakinto the telephone and not hear himself in the earpiece. The telephonecompany transmits the user's voice back for reproduction by theearpiece. This technique is used in the cellular telephony system sothat both sides of a conversation can be heard by monitoring only theforward voice channel.

While legal wiretaps can be performed on cellular telephones, where thetap itself is located at the mobile telephone switching office, thistype of wiretap is not a complete solution to the problem of monitoringthese conversations. If the target mobile telephone is used outside thecoverage area of the tapped MTSO coverage is lost. This problem isespecially troublesome in areas that are located on the boundariesbetween adjacent MTSO's. It is obvious that a "fixed site" wiretap on amobile telephone system cannot be very effective. Therefore, an approachusing the mobile communications medium, namely radio frequencyreception, has significant advantages. The present invention, as will bedescribed further herein, uses such a radio frequency approach.

One prior technique for monitoring cellular telephone conversations issimply to tune a single scanner or receiver to a forward voice channeland listen to the cellular conversation on that frequency (See FIG. 2).Clearly, this is a hit and miss technique as it requires continuousscanning of the forward voice channels to find a specific target mobileunit. This simple approach provides no way of: (a) following the callduring a hand-off; (b) knowing who is talking unless names are spoken;(c) knowing what phone number is being used, other than the fact that itis a cellular telephone; and (d) linking target telephone numbers withcellular conversations (i.e., finding the telephone conversations of thetarget mobile unit when you know the target's telephone number orelectronic serial number). The tape recorder 30 must be controlledmanually and the receiver 32 must be manually tuned. These limitationsprevent effective use of this technique for legal wiretaps by lawenforcement agencies.

A second prior art scheme (FIG. 3) provides the additional feature offollowing the cellular telephone conversation as it shifts from onevoice channel frequency to another, as the target mobile unit passesfrom one cell to another. This embodiment increases the complexity ofthe listening device by the addition of the ability to receive anddecode the digital data bursts that identify the new voice channelassignment occurring on the existing voice channel frequency. Thesedigital data bursts tell the receiver which frequency to tune to next.The two additional functions required to accomplish this task areidentified as the signal processing function 34 and the processingfunction 36 in FIG. 3. The signal processing function 34 converts thefrequency shift keyed (FSK) tone bursts (analog) into digital data byperforming a simple FSK demodulation. The digital data representing thenew voice channel frequency is communicated to the processing function36. Here it is decoded and the processing function 36 then commands thereceiver 32 to tune to the new frequency. In this embodiment theprocessing function 36 is manually tuned to establish the initial voicechannel frequency to which the receiver 32 is to be tuned. Thedisadvantage with this embodiment is the inability to find a mobile unithaving a particular telephone number or electronic serial number. Alsothis technique does not provide the telephone numbers (called orcalling) associated with any call that is being monitored. Anotherlimitation of this technique is its ability to only monitor a singleforward control channel.

The choice of which single forward control channel to monitor iscritical to the probability of intercepting a particular target due tothe unique control channel allocation scheme that is based ongeographical location. Recall that each cell in a cellular telephonesystem operates at only one forward control channel frequency. If themobile target is not located within the coverage area of the cell usingthe monitored forward control channel, then the voice channel assignmentto that target unit will not be available to the intercept system.Therefore, the user of such an intercept system needs prior knowledge ofthe cell in which the target mobile unit is operating. In those fewsituations where the correct forward control channel is the onemonitored and the voice channel assignment is therefore received, thenthe intercept unit retunes its receiver to the commanded forward voicechannel. The intercepter can now monitor the data on the forward voicechannel and is prepared for a cell hand-off when it occurs, as describedabove.

In yet another embodiment (See FIG. 4) more processing power is includedin the processing function 36, providing the ability to store atelephone number that the user is looking for. In this embodimentadditional receiver control is added so that the receiver can first betuned to the forward control channel for receiving telephone numbers andelectronic serial numbers. Once the target number is located, thereceiver locks onto that target mobile unit, starts the tape recorder,and follows the telephone conversation, including cell hand-offs. Thisembodiment requires the entering of the target telephone number and thentuning the receiver 32 to the desired forward control channel. Thistarget telephone number .information is input to the processing function36. The processing function 36 also activates the tape recorder 30 whenthere is a match between the target telephone number and the telephonenumber picked up by the receiver 32.

If the target mobile unit was stationary or at least began its cellularcommunication in the same cell site every time, the FIG. 4 embodimentwould be satisfactory. However, the nature of cellular communications ismobility. The user therefore does not know where a cellular conversationwill originate. The best that can be hoped for is the general vicinityof where that conversation will begin. To solve this problem one couldsimply add many receivers, each having the functions previouslydiscussed in conjunction with FIG. 4. Such a system would include thenumber of receivers necessary to cover the area that the target mobileunit might be in, with each receiver tuned to a different cellularcontrol channel within that area.

With this extension of the FIG. 4 embodiment the system now has thecapability to intercept cellular telephone calls for a fixed number ofcell sites over a geographical area. The limitations of this systeminclude: (a) each receiver must be manually tuned to a designatedforward control channel and there must be some scheme for determiningthe forward control channel of choice; (b) if the target mobile unit hasmoved out of the geographical area, none of the conversations will becaptured; (c) in this scheme the voice channel communications associatedwith a particular target are handled by only that receiver that wasmonitoring the forward control channel containing the voice channelassignment for the target mobile unit, and (d) since there is one taperecorder for every receiver, the tape recorder can only record thoseconversations that the receiver with which it is associated ismonitoring. As a result of this last stated disadvantage, if thecellular intercept user is trying to gather evidence on a particularperson or group of people, that evidence would be spread over several orall the tape recorders. For evidentiary purposes, law enforcementofficials must have all conversations related to a particular person orcase originally recorded on the same machine. For example, a first callof a target mobile unit is monitored by receiver A, the receiver thatwas monitoring the forward control channel when the target's call wasestablished. Sometime after terminating that call, the target makes orreceives another call, but this time, because the target has moved,receiver B is monitoring the forward control channel for the target'snew location and therefore receiver B monitors the call. Since receiversA and B have separate recorders, these two intercepted calls arerecorded on-different recorders.

Another prior art system, shown in FIG. 5, allows simultaneousmonitoring of multiple targets by a single system, having a plurality ofreceivers and a baseband switching matrix. The receivers are designatedwith reference numerals 41A, 41B, 41C, 41D, 41E, and 41F in FIG. 5. Aswitching matrix 42 provides a path to route the audio signal from eachreceiver over independent audio signal lines 43 to a separate dedicatedmonitoring station, identified with reference characters 44A, 44B, and44C. Control signals from a computer 45 via a controller 46 are input tothe switching matrix 42. Control signals are also sent to each receiverfrom a controller 46 over control lines 47. The computer 45 controls theswitching matrix 42 and the receivers 41A through 41F. The capabilitiesof this system are limited by the current configuration for the computer45, which usually include no more than six serial ports, with a singleport required to control each receiver. Thus only six such receivers canbe controlled by the computer 45. Also, the receivers 41A through 41Fand the computer 45 must be collocated to reduce signal deteriorationdue to line length over the control lines 47.

In this FIG. 5 prior art system the audio signal from any receiver canbe routed via the switching matrix 42 to any one of separate dedicatedmonitoring stations 44A through 44C. In practice, the switching matrix42 is controlled so that the audio from the receiver that identifies thetarget mobile unit is recorded on a designated monitoring station andthen all subsequent conversations that are also related to that targetare also recorded on the same monitoring station. In this way theintercept system provides a single tape on which is located allconversations that are relevant to a specific case, where a case is allthe telephone conversations relevant to a single wiretap authorization.The disadvantage, however, is that hardware limitations associated withthe switching matrix 42 limit the number of recording apparatuses toapproximately three. If the system is tracking more than three targetsor cases, there will be multiple case recordings on at least one of therecorders. In practical implementation, the switching matrix 42 and itscontrol are highly complex as any of the receivers 41A through 41F mayat anytime be switched to any of the monitoring stations 44A through44C. The complexity of this prior art system grows exponentially morecumbersome with the addition of each receiver and monitoring station.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations discussed above bydynamic reallocation of receiver resources, by interconnection of thereceivers with a network, and by including a processing function in eachreceiver. The system offers judicious use of data exchange between thesystem controller and each receiver over a high bandwidth local areanetwork and a scheme for dynamically reallocating the receiverresources. With sufficient network bandwidth and controller processorhorsepower, the intercept system of the present invention can beexpanded to twenty or more channels, as many or few as are needed tocover the entire city or just a part of it. In addition, each of thesechannels can be a monitoring station for a unique target so that twentyor more targets can be separately monitored and recorded. This expansionalso does not require any architecture modifications or hardwarereconfiguration, it simply requires adding more receivers to the poolwith the necessary software to control them. By putting processingcapabilities in each receiver, the load on the system controller isreduced, thus allowing the use of a simpler and less costly controller.

The inventive solution to the disadvantages identified in the prior artsystems above involves adding to the plurality of receivers a commonpoint of data entry that has intelligence to know which receiver istuned to which frequency, which tape recorder is handling which cellulartelephone numbers, the ability to query a device in the system to learnwhat forward control frequencies are available to listen to, and to actas a central collection point for all data coming in from the receiversregarding voice assignments. With this single-point control capabilitythe user can route cellular conversations pertaining to a particularperson or group to a specific tape recorder by retuning a specificreceiver to the voice channel of the target mobile unit so thatconversation can be recorded on the tape recorder identified with thattarget. Further, this system can follow the target mobile unit throughdifferent cell areas by querying the device for active forward controlchannels on a regular basis and retuning receivers to one of those newcontrol channels as required.

In addition to the single point of data entry and control, a cellularintercept system of the current invention incorporates a networkarchitecture. By using this method of connecting the receivers severaladvantages are gained. The number of receivers to be connected isunlimited. If the user is targeting a small cell area only a fewreceivers are needed, but if the targeting area is large, like a largecity, the user can connect as many receivers as there are cell sites.The network architecture, with its predefined standards and protocols,allows the receivers to be remotely located from the controller, whichis a necessity for covering large areas due to the reuse of certainfrequencies within the cellular system as the area of interest grows.The use of a token passing network also allows the receivers to initiatecommunications with the controller. This is an important distinctionfrom simple wire connections or a bus architecture where the controllermust query each receiver to get data from that receiver. A busarchitecture also has the disadvantage of limiting the number ofreceivers and the distance those receivers can be located away from thecontroller.

In the present invention the system controller communicates with theplurality of receivers, where several (or all) of the receivers are alsoconnected to monitoring apparatuses, for example a recorder. Eachreceiver monitors a different cell, or more specifically, the basestation page from a different cell in the geographical area of interest,and decodes the telephone number or electronic serial number in thepage. The decoded information is input to the controller and when a pageidentifies a target mobile unit (by telephone number or electronicserial number), the system controller recognizes this and commands oneof the receivers to retune to the forward voice channel that was justassigned to the target mobile unit in the page. The retuned receiver isthat receiver which is physically connected to the monitoring (e.g.recording) station dedicated to that particular target, i.e., themonitoring station for all communications related to a particular case.Once retuned, the receiver is removed from the pool of receiversmonitoring forward control channels. The retuned receiver monitors theforward voice channel of the target mobile unit and follows cellhand-offs as they occur. The system controller may then retune one ofthe other receivers in the pool to cover monitoring of the forwardcontrol channel that the recently retuned receiver had been monitoring.This retuning would be based on a pre-programmed priority scheme. Forinstance, the highest priority target monitoring station may bephysically connected to the receiver that is assigned to monitor thelowest priority cell's forward control channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can be more easily understood and the furtheradvantages and uses thereof more readily apparent, when considered inview of the description of the preferred embodiments and the followingfigures in which:

FIG. 1 is a diagrammatical presentation of a cellular telephone system;

FIGS. 2-5 are block diagrams of prior art cellular intercept systems;

FIG. 6 is a block diagram of a cellular intercept system constructedaccording to the teachings of the present invention; and

FIGS. 7, 8, and 9 are diagrams of details of the cellular interceptsystem of FIG. 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As discussed above, in a cellular telephony system many discrete,limited coverage base transceivers (base stations 16 in FIG. 6) arenetworked into a system that provides widespread coverage over ageographical area. The connection between the mobile unit 14 and thebase station 16 is established by a global page from each base station16, on its unique forward control channel, in the MTSO network. Thisglobal page identifies the called mobile unit 14 by either a telephonenumber or the electronic serial number. Upon receipt of the global page,the called mobile unit 14 responds, on the reverse control channel, tothe base station 16 of the cell in which it is located to identify itspresence in the cell. The receiving base station 16 then responds on theforward control channel with a base station page that identifies thevoice channel assignment for the called mobile unit. The base station 16in that cell then handles the traffic on the forward and reverse voicechannels.

Intercepting telephone conversations of a particular target mobile unitcan be accomplished by first intercepting the voice channel assignment.Since this channel assignment is available only by intercepting the basestation page to the mobile unit 14, the cellular intercept system needsto know the location of the mobile unit 14 and thus the proper cell 12to monitor. Alternatively, the intercept system must monitor multiplecells to cover a large geographical area, which increases the likelihoodof intercepting the appropriate base station page. To monitor severalcells in a geographical area requires multiple receivers, where each iscapable of identifying the control data associated with a target mobileunit and the voice channel assignment for that target mobile unit. Thisfeature is one of the key aspects of the present invention.

FIG. 6 is a block diagram of a cellular intercept system 70 includingreceivers 71, 72, 73 and 74 under the control of a system controller 78.While FIG. 6 shows four receivers, it is to be understood by thoseskilled in the art that the cellular intercept system 70 can beincreased in size and coverage area to include any number of receivers,as many as twenty or more given the current processing power associatedwith the system controller 78. Each receiver 71, 72, 73 and 74 isconnected to an antenna 82, 84, 86 and 88, respectively, for receivingcommunications from one of the base stations 16 and from mobile units14. The system controller 78 communicates with each receiver 71 through74 via an interconnect 90. In the preferred embodiment this interconnectis a token-passing local area network having a bandwidth greater thanapproximately 200 kb/s, e.g., Ethernet or Appletalk. Access is gained tothe network by the well-known token-passing scheme. Each receiver isalso connected to a monitoring station, shown in block diagram form inFIG. 6 and identified by reference characters 92, 94, 96 and 98. Amonitoring station may include a listening device, e.g., headphones,and/or a recording apparatus. Each monitoring station may also bepreassigned to a specific target or case. For example, the monitoringstation 92 may be designated to monitor and/or record conversationsassociated with target A, independent of which receiver 71 through 74receives the base station page intended for target A. To accomplishthis, the system controller 78 controls the receiver 71 to always assignthat receiver and its associated monitoring station 92 to all telephoneconversations involving target A.

FIG. 7 shows a block diagram of the receiver 71. This block diagram istypical for all receivers in the cellular intercept system 70. Radiofrequency signals collected by the antenna 82 are serially processed byan RF front-end 100 and a demodulator 102. The RF front-end 100 and thedemodulator 102 incorporate the well-known classical circuitry forperforming these functions. The demodulator 102 is capable ofdemodulating narrowband frequency modulated signals used in the cellularsystem. The baseband signal from the demodulator 102 is input to themonitoring station 92 for monitoring and/or recording, as shown in FIG.7. Immediately prior to a cell hand-off, the new voice channelassignment for the target mobile unit is transmitted over the existingvoice channel in the form of audio tones. These audio tones, afterpassing through the RF front-end 100 and the demodulator 102, are inputto a signal processor 104 for decoding. The digital representation ofthat new voice channel frequency is input to a controller 106. Thecontroller 106, which is for example a microprocessor, signals the RFfront-end 100 to retune to the new voice channel assignment so thatmonitoring of the cellular telephone conversation can continue on thenew voice channel. Information regarding this change in receivingfrequency is also sent to the system controller 78 over the interconnect90. The system controller 78, which is also a microprocessor forexample, checks the new voice channel assignment to be sure it is avalid voice channel frequency within the geographical area. The systemcontroller 78 also determines whether other conversations are alreadybeing monitored on that same voice channel frequency, and if the otherconversations have a higher priority, can command the receiver 71, viathe controller 106, to discontinue monitoring activities on that voicefrequency.

The basic intent of the communications; intercept system 70 is the useof intelligent receivers to minimize network traffic between the systemcontroller 78 and the receivers 71-74. Further, receiver pooling devotesa quantity of n receivers to monitor the base station pages from ncells, with receivers individually removed from the pool, as needed, tomonitor conversations of target mobile units. Although only fourreceivers are illustrated in FIG. 6, it is understood that the systemcan be increased to include as many as twenty (or more) receivers forincreased monitoring capabilities. In setting up the communicationsintercept system 70, the cells to be monitored are chosen so as to forma coverage net around a target mobile unit's most likely geographiclocation. The global pages from each receiver 71, 72, 73 and 74 arepassed to the system controller 78. Further, after the called mobileunit 14 responds to a global page, the base station 16 transmits a basestation page containing the voice channel assignment for the call. Thischannel assignment will also be received by the receiver monitoringpages in that cell 12, demodulated, decoded, and sent to the systemcontroller 78. Further, prior to system operation, the telephone numbers(or electronic serial numbers) of the target mobile units are enteredinto the system controller 78 so that a comparison can be made betweenthe target telephone numbers and the telephone numbers received in theglobal pages. When a match occurs between these telephone numbers, thesystem controller 78 designates one of the receivers, for examplereceiver 71, as the intercept receiver. The receiver 71 is thencommanded by the system controller 78 to retune to the forward voicechannel that had been identified in the base station page. Once retuned,receiver 71 demodulates the telephone conversations of the target mobileunit (as carried on the forward voice channel). The conversation can bemonitored and/or recorded at the monitoring station 92, which isdedicated to the receiver 71. In one embodiment, once a receiver isremoved from the receiver monitoring pool, the remaining receivers areretuned to always monitor the strongest forward control channels.

The basic aspects of the operation of the cellular intercept system 70,especially the interaction between the receivers 71 through 74 and thesystem controller 78, are shown diagrammatically in FIG. 8. Allsignal/commands from the system controller 78 shown in FIG. 8 are inputto the controller 106 (See FIG. 7) of the receiver 71. The controller106 then commands the various functions of the receiver 71 to accomplishthe intended result.

The system controller 78 first commands one or more of the n receivers(in this example receiver 71) to tune to a control channel x for thepurpose of receiving global pages. All the commands illustrated in FIG.8 as passing between the system controller 78 and the receiver 71 arepassed over the interconnect 90. The receiver 71 responds with anacknowledgement and then the system controller 78 identifies one or morespecific telephone numbers that the receiver 71 is to be searching for.The receiver 71 parses large quantities of data in the global pages insearch of the specified telephone number. Turning to FIG. 7, this isaccomplished by the reception and demodulating of the RF signal in whichthe called telephone number is encoded. This analog information isdecoded in a signal processor 104 and then sent to the controller 106.The controller 106 compares the transmitted or called telephone numbersthat are received from the signal processor 104 with the contents of itsmemory, i.e., the list of target telephone numbers sent from the systemcontroller 78. Note that the system controller 78 can send a differentlist of telephone numbers to be searched for to each receiver, in thisway expanding the capabilities of the system.

When a match is found the receiver 71 acknowledges this by sending amessage from the controller 106 to the system controller 78. If thetarget mobile unit having the specified telephone number received theglobal page, the same global page that receiver 71 received, the mobileunit will respond, and then the base station 16 in the affected cell 12will transmit a base station page providing the voice channel frequencyassignment. The receiver 71 will also receive the base station page andsignal the system controller 78 that the "phone call will take place onchannel ABC." With this information in hand, the system controller 78commands the receiver 71, or any of the other n receivers in the pool,to tune to channel ABC for the purpose of monitoring the target'scellular telephone conversation. As described earlier, the receiver 71also follows the cell hand-offs as they occur and informs the systemcontroller 78 of these occurrences.

The receiver 71 also informs the system controller 78 that the monitoredtelephone call continues in progress and whether any state changes haveoccurred. Examples of such state changes include the occurrence of acell hand-off or a change in transmitted power level of the targetmobile unit. The base station 16 continually monitors the power level ofeach mobile unit 14 and commands that mobile unit to either increase ordecrease the transmitted power level as required to maintain efficientcommunications. As the need arises, the system controller 78 responds tothe receiver 71 with change of state message responses, confirming, forexample, a cell hand-off. When a call ends, a message to this effect issent to the system controller 78, followed by a response to the receiver71 to tune to a control channel y for receiving global pages andstarting the process over again.

FIG. 9 is a diagrammatic representation showing operation of thecontroller 106, i.e., a controller within one of the receivers 71through 74. At power up the controller 106 enters a wait state 120. If acommand is received, as shown by step 124, processing continues via path126. If no command is received the controller remains in wait state 120via the return path 128. Following path 126, there is a step 130 wherethe command is filtered to determine whether it is a "search for atelephone number" command. If it is not such a command, then processingmoves to step 131 where the command is processed.

If the command is a "search for telephone numbers" command processingmoves to a step 132 where the controller 106 parses all the messagesfrom the demodulator 102, decodes them, and passes them to the signalprocessor 104. As discussed above, the signal processor 104 accepts theanalog signal from the demodulator and converts it to a digital signalrepresenting the telephone numbers received. At step 134 query is madeas to whether the received telephone number is one of interest. Thisfunction is accomplished in the controller 106, where the receivedtelephone number is compared with target numbers received from thesystem controller 78 and stored in the controller 106. If a matchoccurs, processing moves to a step 136 where the received telephonenumber is passed to the system controller 78. Otherwise, processingmoves to a step 138. If a stop processing command has been received bythis time then processing moves back to the wait state 120. If a stopprocessing command has not been received, then the controller 106continues to parse the received telephone numbers and determine whetherone is a telephone number of interest at the step 134.

As mentioned above and as represented by step 131 in FIG. 9, there areother commands that the controller 106 processes. The reset or power upcommand is used to mute the receiver 71 at the beginning of a session,for example. At this time the RF front end 100 can be set to apredetermined frequency or the system controller 78 may issue a commandidentifying the initial frequency. The receiver 71 can also be commandedto listen to information on various frequencies, including the forwardcontrol channel, the forward voice channel, and the reverse controlchannel. With respect to the forward control channel, the receiver 71can also be further commanded to listen only to page data or only tovoice channel assignment frequencies. The system controller 78 can alsoissue commands to change the audio output level from the demodulator 102or to mute that output altogether. On command from the system controller78, the signal processor 104 can decode power level information obtainedfrom any mobile unit 14.

As previously discussed in conjunction with FIG. 8, at this point thesystem controller 78 responds to the telephone numbers received and thefollow up voice channel assignment by tuning one of the receivers tothat voice channel for the purpose of intercepting the cellulartelephone conversation.

One key feature of the present invention is the pooling of n receiversto receive both the global pages and the base station pages and thedesignation of one of those n receivers as the telephone trafficintercept receiver for a particular target. In fact, this designationcan be made before any receiver receives page data for that targetmobile unit. In any case, when one of the receivers is removed from thepool to monitor and/or record the telephone conversations of the targetmobile unit, the remaining receivers continue monitoring global pages insearch of telephone numbers for other target mobile units, asillustrated by the example in FIG. 8. In this way, the system operationis optimized so that the maximum number of receivers is alwaysmonitoring global pages and as each target mobile unit is found, onereceiver drops out of the pool to monitor/record conversations of thattarget mobile unit.

In the communications intercept system 70 note that the systemcontroller 78 can choose any receiver 71 through 74 to be the monitoringreceiver for conversations from any target mobile unit, and thus ineffect can have that target mobile unit conversation monitored on anyone of the monitoring stations 92, 94, 96, and 98. This is especiallyhelpful in establishing a case for the wiretap authorities in that allconversations for a particular target unit can be routed to a designatedmonitoring station so that all those conversations are monitored andrecorded in sequence.

The system controller 78 can also command any receiver, for examplereceiver 71 via the controller 106, to tune to a reverse data channel insearch of a called telephone number. Once such a called number isidentified, the system controller 78 can command any receiver, forexample the receiver 71 via the controller 106, to tune to the base pagefrequency for the cell in which the calling mobile unit is located. Thevoice channel assignment for the cellular telephone conversation will betransmitted on the base station page frequency and received by thereceiver 71. The controller 106 can then retune the receiver 71 to thatvoice channel frequency and also advise the system controller 78 of thisfrequency assignment.

As can be seen, the architecture of the communications intercept system70 easily allows for the expansion of the number of monitoringreceivers. Also, if certain receivers are designated as trafficintercept receivers, the number of these can also be expanded. The onlylimitations on the expansion of the communications intercept system 70are the capacity of the interconnect 90 and the processing power of thecontroller 78.

In another embodiment of the present invention each receiver 71 through74 would broadcast to all other receivers on the network 90 that atelephone number match has occurred between the telephone numbertransmitted in the global page and the list of target telephone numbersin that receiver's memory. The receiver on the network 90 that isidentified as the monitoring and/or recording receiver for thattelephone number would then respond back to the initiating receiver witha message that the identified receiver will retune to the voice channelfrequency for that cellular telephone conversation and record it. Thiswould be followed by a message to the system controller 78 as to whathas occurred. In this embodiment the system controller 78 is relegatedto a data entry, display, and overall system configuration role becausethe receivers 71 through 74 are capable of controlling their ownsub-systems and communicating with the other receivers in the cellularintercept system 70. Such a system of distributed control furtherreduces traffic carried on the interconnect 90.

While we have shown and described embodiments in accordance with thepresent invention, it is understood that the same is not limited theretobut is susceptible to numerous changes and modifications as known to aperson skilled in the art, and we therefore do not wish to be limited tothe details shown and described herein, but intend to cover all suchchanges and modifications as are obvious to one of ordinary skill in theart.

What is claimed is:
 1. A cellular communications intercept system forintercepting cellular telephone communications on a cellular networkbetween a base station and a target mobile unit, wherein the cellularnetwork includes a plurality of cells with each cell having a basestation, wherein all mobile units communicate with the base station onan assigned voice channel, and wherein each mobile unit has apreassigned unique identification number, and wherein a global page on aforward control channel from each base station to all mobile units inthe cellular network includes the identification number of the calledmobile unit, and wherein a base station page on the forward controlchannel of the base station for the cell in which the called mobile unitis located identifies the voice channel for the called mobile unit, saidcellular communications intercept system comprising:a plurality oftunable receivers, wherein each receiver is tuned to the forward controlchannel of one base station for receiving the base station pages and theglobal pages transmitted on that forward control channel; wherein eachof said plurality of receivers includes decoder means responsive to eachreceived global page for extracting therefrom the identification numberof the called mobile unit; wherein each of said decoder means isresponsive to each base station page for extracting therefrom the voicechannel assignment for the call to the mobile unit; storage means forstoring target mobile unit identification numbers; comparator meansresponsive to the identification number of the called mobile unit andresponsive to the stored target mobile unit identification numbers andfor producing a match signal when the identification number of a calledmobile unit matches one of the stored target mobile unit identificationnumbers; controller means for controlling said plurality of receiversand responsive to said match signal and said voice channel assignment;wherein one receiver of the plurality of receivers is designated as theprimary receiver for at least one predetermined target mobile unit, andwherein the controller means, in response to the match signal, commandssaid identified receiver to tune to the assigned voice channel for saidat least one predetermined target mobile unit, such that voicecommunications for said at least one predetermined target mobile unitare received by said identified receiver; and a monitoring stationconnected to each one of the plurality of receivers for monitoring allcommunications intercepted by said connected receiver.
 2. The cellularcommunications intercept system of claim 1 wherein the remainingreceivers of the plurality of receivers are retuned, as may be required,to the highest priority forward control channels in the geographic area.3. The cellular communications intercept system of claim 1 wherein theidentification number is the telephone number of the target mobile unit.4. The cellular communications intercept system of claim 1 wherein theidentification number is the electronic serial number of the targetmobile unit.
 5. The cellular communications intercept system of claim 1wherein under control of the controller means each of the plurality ofreceivers can be tuned to a different voice channel assignment such thatthe communications intercept system can simultaneously monitor thecommunications of more than one target mobile unit.
 6. The cellularcommunications intercept system of claim 1 wherein the system controllerand the plurality of receivers are interconnected by a local areanetwork.
 7. The cellular communications intercept system of claim 6wherein each of the plurality of receivers is located at a site remotefrom the controller means.
 8. The cellular communications interceptsystem of claim 1 wherein the monitoring station includes a recordingdevice.
 9. A cellular communications intercept system for interceptingcellular telephone communications on a cellular network between a basestation and a plurality of target mobile units, wherein the cellularnetwork includes a plurality of cells with each cell having a basestation, wherein each one of said plurality of target mobile unitscommunicates with a base station on an assigned voice channel, andwherein each one of said plurality of target mobile units has apreassigned unique identification number, and wherein a global page on aforward control channel from each base station to all mobile units inthe cellular network includes the identification number of the calledmobile unit, and wherein a base station page on the forward controlchannel of the base station for the cell in which the called mobile unitis located identifies the voice channel assignment for the called mobileunit, said cellular communications intercept system comprising:aplurality of tunable receivers, wherein each receiver is tuned to theforward control channel of one base station in the cellular network forreceiving the global page on that forward control channel; wherein eachof said plurality of receivers includes decoder means responsive to eachreceived global page for extracting therefrom the identification numberof the called mobile unit; wherein the identification number of at leastone of said plurality of target mobile units is stored within each ofsaid plurality of receivers; wherein one receiver of said plurality ofreceivers is designated as the monitoring receiver for each targetmobile unit; wherein when any one of said plurality of receiversidentifies a match between the identification number of the calledmobile unit and the identification number of one of said plurality oftarget mobile units, said identifying receiver produces a match signalthat is communicated to each of the other plurality of receivers;wherein in response to said match signal said receiver of said pluralityof receivers having been designated as the monitoring receiver for theidentified target mobile unit responds thereto and retunes to the voicechannel for the identified target mobile unit for the purpose ofmonitoring the communications associated with the identified targetmobile unit.
 10. A cellular communications intercept system forintercepting cellular telephone communications on a cellular networkbetween a base station and a target mobile unit, wherein the cellularnetwork includes a plurality of cells with each cell having a basestation, wherein all mobile units communicate with the base station onan assigned voice channel, and wherein each mobile unit has apreassigned unique identification number, and wherein a call is placedby a mobile unit by transmitting the called telephone number on thereverse control channel to the base station of the cell in which themobile unit is located, and wherein when the call is set up the basestation transmits to the calling mobile unit a base station page on itsforward control channel that identifies the assigned voice channel forthe call, and wherein in response to the base station page, the callingmobile unit uses the assigned voice channel for the call, said cellularcommunications intercept system comprising:a plurality of tunablereceivers, wherein each receiver is tuned to the reverse control channelof one base station for receiving the called telephone numberstransmitted on that reverse control channel; wherein each of saidplurality of receivers is responsive to a base station page forextracting therefrom the voice channel assignment for the call; storagemeans for storing target telephone numbers; controller means forcontrolling said plurality of receivers; wherein the called telephonenumbers are input to said controller means; wherein the voice channelassignments are input to said controller means; wherein one receiver ofthe plurality of receivers is designated as the primary receiver for atleast one target mobile unit; and wherein when the called telephonenumber matches a target telephone number, said controller means causessaid primary receiver to tune to the voice channel assignment for thecall for monitoring the telephone call.